1. Field of the Invention
The present invention relates to a semiconductor device with an overcurrent protection circuit that protects an output MOS transistor against overcurrent.
2. Description of Related Art
Automobiles, home electric appliances and the like use power devices to regulate large current or large voltage. In the power devices, extraordinarily large current (overcurrent) may flow through an output MOS transistor due to load short circuit or other trouble, causing the output MOS transistor to break. Thus, what is called an IPD (Intelligent Power Device) has been in use; the IPD includes an overcurrent protection circuit that protects an output MOS transistor against overcurrent.
FIG. 3 illustrates a block diagram of an overcurrent protection circuit for a semiconductor device according to prior art. The semiconductor device 101 includes, as illustrated in FIG. 3, an output MOS transistor M102 and an overcurrent protection circuit 104. The output MOS transistor M102 has a drain connected to a power source terminal 108, a source connected to an output terminal 109 and a gate connected to a charge pump 106 via a resistor R105. Power source voltage (VBB) of a battery, for example, is applied to the power source terminal 108.
The overcurrent protection circuit 104 includes a sense MOS transistor M103, control MOS transistor M110 and sense resistor Rs120. The sense MOS transistor M103 detects an output current flowing through the output MOS transistor M102, and its gate and drain are connected to the gate and drain of the output MOS transistor M102, respectively. The sense resistor Rs120 converts a sense current Isens of the sense MOS transistor M103 into a sense voltage. The control MOS transistor M110 detects an overcurrent state by use of the sense voltage and limiting the output current flowing through the output MOS transistor M102.
As the sense MOS transistor M103, a MOS transistor is used which is different in the number of cells from the output MOS transistor M102 but has the same structure. The sense current Isens corresponding to a cell number ratio relative to the output MOS transistor M102 is made to flow through the sense MOS transistor M103, whereby the output current flowing through the output MOS transistor M102 is monitored with high accuracy. The sense current Isens flowing from the sense MOS transistor M103 produces a potential difference across the sense resistor Rs120. This potential difference is proportional to the output current flowing through the output MOS transistor M102. Using the potential difference, the gate-source voltage of the output MOS transistor M102 which depends on a drain-source resistance of the control MOS transistor M110 is varied, whereby the output current is regulated.
FIG. 1(B) of Japanese Patent Laid-Open Publication No. HEI 5-235365 (Patent Document 1) and FIG. 1 of Japanese Patent Laid-Open Publication No. 2006-303843 (Patent Document 2) disclose a circuit configuration similar to the above described semiconductor device.
Recently, the area of semiconductor chip has been increasingly reduced to achieve cost reduction. Accordingly, permissible breakdown voltage of the output MOS transistor has been increasingly small. Thus, the output MOS transistor is more likely to break than before, so there are strong demands for a technique for achieving higher accuracy in the function of regulating the output current.
According to the prior art, in the mechanism of varying the gate-source voltage of the output MOS transistor M102, i.e., of varying the drain-source resistance of the control MOS transistor M110, variations in electrical characteristics in both the sense resistor Rs120 and the control MOS transistor M110 should be considered. The variations significantly affect a variation in circuit characteristic, that is, a variation in output current limiting value.
Factors affecting the drain-source resistance value of the control MOS transistor M110 include gate-source voltage Vgs of the control MOS transistor M110, the threshold voltage of the control MOS transistor M110 and the drain-source voltage of the control MOS transistor M110. Here, the drain-source voltage of the control MOS transistor M110 is equal to the gate-source voltage of the output MOS transistor M102; and when these voltages are assumed to be constant, the factors affecting the drain-source resistance value of the control MOS transistor M110 are the gate-source voltage Vgs of the control MOS transistor M110 and the threshold voltage of the control MOS transistor M110.
The gate-source voltage Vgs of the control MOS transistor M110 is expressed by the following <formula 1>.Vgs(M110)=Rs×Isens   <formula 1>where Isens is a sense current outputted from the sense MOS transistor M103 and Rs is a resistance of the sense resistor Rs120. Here, when Isens is assumed to be constant, a variation in resistance Rs of the sense resistor Rs120 becomes a main factor.
A variation in threshold voltage of the control MOS transistor M110 occurs independently of a variation in resistance Rs of the sense resistor Rs120. Thus, if the variation in resistance Rs of the sense resistor Rs120 and the variation in threshold voltage of the control MOS transistor M110 are not tightly controlled, the variation in output current limiting value may increase.